Marine power distribution and propulsion systems are well known. In a typical arrangement a series of power converters are used to interface a main ac bus to a series of loads which can be electric motors, e.g., propulsion motors or thrusters. Other loads can be connected directly to the main ac bus or connected to an auxiliary ac bus which is in turn connected to the main ac bus by means of a transformer. The ac buss typically operate at different voltages, e.g., 690 VAC and 440 VAC.
Each power converter can have an ‘active front end’ (AFE) converter with a supply-side active rectifier/inverter (or ‘front end’ bridge) having ac terminals connected to the main ac bus and a load-side active rectifier/inverter connected to the load. The dc output of the supply-side active rectifier/inverter is connected to the dc input of the load-side active rectifier/inverter by a dc link. In normal operation, the supply-side active rectifier/inverter will operate as an active rectifier to supply power to the dc link and the load-side active rectifier/inverter will operate as an inverter to supply power to the load.
Each active rectifier/inverter will typically have a conventional topology.
A prime mover (e.g., a diesel engine or turbine) is connected to individual generators which supply power to the main ac bus.
The main ac bus can be equipped with protective switchgear with circuit breakers and associated controls.
The marine propulsion system will typically include a first (or port) ac bus and a second (or starboard) ac bus that are interconnected by a bus tie. Some marine propulsion systems use a plurality of ac bus sections or groups interconnected by a plurality of bus ties to improve power availability.
In one arrangement, shown in FIG. 1, the marine propulsion system 1 includes a power take-in/power take-out (PTI/PTO) hybrid drive system 2. The hybrid drive system 2 includes an electrical machine 4 and a diesel engine 6. The electrical machine 4 can be a synchronous machine or an asynchronous machine such as an induction machine. The rotor of the electrical machine 4 and the driving end of the diesel engine 6 are mechanically coupled for example through a clutch and gearbox 8 and are used to drive a propulsion thruster 10, for example. The electrical machine 4 is connected to the main ac bus 12 by means of an AFE converter 14 with a supply-side active rectifier/inverter 16 having ac terminals connected to the main ac bus and a machine-side active rectifier/inverter 18 connected to the electrical machine. The dc output of the supply-side active rectifier/inverter 16 is connected to the dc input of the machine-side active rectifier/inverter 18 by a dc link 20. During a PTI mode, the electrical machine 4 is operated as a motor and is used to drive the propulsion thruster. Power is supplied to the electrical machine 4 from the main ac bus 12 through the AFE converter 14 with the supply-side active rectifier/inverter 16 being operated as an active rectifier and the machine-side active rectifier/inverter 18 being operated as an inverter. During a PTO mode, the electrical machine 4 is operated as a generator with the rotor of the electrical machine being driven by the diesel engine 6. Power is supplied from the electrical machine 4 to the main ac bus 12 through the AFE converter 14 with the machine-side active rectifier/inverter 18 being operated as an active rectifier and the supply-side active rectifier/inverter 16 being operated as an inverter.
Diesel generators 22, 24 supply power to the main ac bus 12. In some situations, the hybrid drive system 2 can be used as the only power source for the marine propulsion system 1 during a normal operating mode. For example, the diesel generators 22, 24 can be turned off to reduce fuel costs or to try and minimise harmful emissions emitted the diesel engines. In other situations, the hybrid drive system 2 can be the sole power source for the marine propulsion system 1 because the diesel generators 22, 24 are non-operational for any reason, e.g., as a result of an electric power blackout. In this case, the hybrid drive system 2 might need to recover the main ac bus 12 by supplying power to the main ac bus through the AFE converter 14. In other words, the electrical machine 4 will be driven by the diesel engine 6 and operated as a generator for recovery purposes.
In a situation where the hybrid drive system 2 is the sole power source for the marine propulsion system 1, and a short-circuit fault occurs at a certain load branch of the marine propulsion system 1, the hybrid drive system 2 must operate without tripping to avoid an entire power blackout. To ensure that the circuit breaker associated with the load branch that is experiencing the short-circuit fault is able to properly discriminate and disconnect the faulty load branch from the remainder of the marine propulsion system 1, the AFE converter 14 needs to provide a certain amount of overcurrent for a certain period of time to enable selective fault discrimination. The AFE converter 14 must also be capable of operating without tripping or failing due to thermal overload. In particular, one or both of the supply-side active rectifier/inverter 16 and the machine-side active rectifier/inverter 18 may need to provide overcurrent and perhaps at a level that is determined by the X/R ratio during the short-circuit fault.
One way of limiting fault current in such situations is to use reactors or transformers. However, this is a passive strategy and is highly dependent upon the impedance of the reactors or transformers. The passive impedance is able to limit the amplitude of the fault current, but cannot eliminate the imbalance caused by individual phase fault currents and any dc offset between individual phase and multi-phase fault currents. Imbalanced fault currents can lead to system instabilities, additional negative sequence voltages and currents, increased thermal stress on the system components etc. The dc offset can cause wound magnetic components (e.g., in the transformers) to saturate and consequently lead to cascaded system failures. A passive strategy has other drawbacks such as high cost, large physical size and mass, additional voltage drops and high standby losses.
An embodiment of the present invention proposes an alternative method of current limitation which avoids fault current imbalances and dc offsets and does not need additional passive hardware components in the system.